Say when you do not know whether you can follow through.

Uncertainty spoken early is useful; uncertainty hidden becomes a broken promise later. When you say yes to keep the moment comfortable, you hand someone a plan built on something you were never sure of. A clear maybe lets them arrange a backup while there is still time. Most people mind an honest doubt far less than a late collapse.

Think about anything you have half agreed to. Go back and say the true version: what you can probably do, what depends on something else, and when you will know. The awkward sentence now saves a worse one later.